1. Field of the Invention
The present invention relates to a low-loss switching power supply circuit that has overcurrent characteristics.
2. Description of the Related Art
A conventional switching power supply circuit is shown in FIG. 9. Referring to FIG. 9, a switching power supply circuit 1 is a self-excited switching power supply circuit, and is comprised of a direct-current power supply 2, a transformer 3 having a primary winding 4, a secondary winding 5, and feedback winding 6 wound around the transformer 3, a FET 7 as a switching element, a control circuit 8, a rectifying circuit 9, and a starting resistor 10. Herein, one end of the primary winding 4 is connected to one end of the direct-current power supply 2, and the other end is connected in series with the other end of the direct-current power supply 2 through the intermediary of the FET 7. One end of the feedback winding 6 is connected to a gate 7a as the control terminal of the FET 7, through the intermediary of the control circuit 8. The rectifying circuit 9 is connected to the secondary winding 5. Also, the one end of the primary winding 4 is connected to the connection point between the gate 7a of the FET 7 and the control circuit 8 through the intermediary of a starting resistor 10. Herein, the resistance value of the starting resistor 10 varies depending on the voltage of the direct-current power supply 2, but is, for example, about 390 kxcexa9 if the voltage of the direct-current power supply 2 is 140 V.
In the switching power supply circuit 1 described, firstly a bias current flows from the direct-current power supply 2 into the gate 7a of the FET 7 through the starting resistor 10. Since there is an internal capacitance between the gate and the source of the FET, the potential of the gate 7 rises in a given time, and the FET 7 conducts at the point in time when the threshold value of the FET is exceeded, so that a current begins to flow through the primary winding 4. Thereafter, the FET 7 alternately conducts and switches off repeatedly by means of the control circuit 8, and thereby an alternating voltage is induced in the secondary winding 5. By rectifying this alternating voltage by means of the rectifying circuit 9, an output of the direct-current voltage is obtained.
Incidentally, in the switching power supply circuit 1, the voltage of the direct-current power supply 2 is not always constant, but can vary. For example, if the voltage of the direct-current power supply 2 decreases, the current flowing into the gate 7a of the FET 7 through the starting resistor 10 decreases, and consequently the time that elapses before the FET 7 conducts increases, in other words, the starting time of the switching power supply 1 becoming longer. In contrast, if the voltage of the direct-current power supply 2 increases, the current flowing into the gate 7a of the FET 7 through the starting resistor 10 increases, and consequently the time that elapses before the FET 7 conducts decreases, in other words, the starting time of the switching power supply 1 becoming shorter. In addition to that, since the current flowing through the starting resistor 10 increases, the loss in the starting resistor 10 also increases. In general, there is a design need so that the FET conducts in a given starting time even if the voltage of direct-current power supply 2 is low, and hence the resistance value of the starting resistor 10 is set to a relatively low value. As a consequence, the current flowing through the starting resistor increases not only when the voltage of the direct-current power supply 2 is high, but also when it is an average voltage, which results in a considerably high loss.
In relation to such a problem, Japanese Unexamined Utility Model Publication No. 61-13586, and Japanese Unexamined Patent Publication No. 10-108462 have disclosed a structure wherein a constant-current circuit is provided between a starting resistor and the control terminal of a switching element. By disposing the constant-current circuit between the starting resistor and the control terminal of the switching element, it is possible to maintain constant the bias current flowing into the control terminal of the switching element irrespective of variations in the voltage of the direct-current power supply, and to maintain constant the starting time of a switching power supply and the loss in the starting resistor.
However, there is a problem that, in the structure wherein a constant-current circuit is provided between a starting resistor and the control terminal of a switching element, a bias current flows through the starting resistor, and hence it is impossible to avoid the loss in the starting resistor. Further, there is a problem that, in the case where an output is short-circuited when the voltage of the direct-current power supply is high, the occurrence of a large bias current shortens the starting time, making the switching element generate heat, and thus making it susceptible to damage.
The present invention can solve the aforementioned drawbacks associated with the conventional art and provides a switching power supply circuit capable of maintaining constant the current flowing into the control terminal of a switching element, reducing the loss, and further preventing the failure of the switching element when an output is short-circuited.
The switching power supply circuit of the present invention comprises: a transformer having a primary winding, a secondary winding, and a feedback winding; a direct-current power supply of which one end is connected to one end of the primary winding; a switching element connected in series between the other end of the direct-current power supply and the other end of the primary winding; a control circuit provided between the control terminal of the switching element and the feedback winding; a rectifying circuit connected to the secondary winding; and a constant-current circuit provided between the one end of the primary winding and one of the control circuit and the control terminal of the switching element.
According to the invention, it is possible to maintain constant the bias current flowing into the control circuit or the control terminal of the switching element irrespective of variations in the voltage of the direct-current power supply, and to maintain constant the starting time of the switching power supply circuit. Furthermore, it is also possible to avoid the loss to be caused by the current flowing through the starting resistor, because of the absence of the starting resistor connecting the one end of the primary winding with the control terminal of the switching element.
In addition, even if an output is short-circuited when the voltage of the direct-current power supply is high, the bias current flowing into the switching element or the control circuit is constant, and therefore the starting time is not shortened. Hence, the switching element does not generate more heat than is necessary, which permits the prevention of failure of the switching element.
For the purpose of illustrating the invention, there is shown in the drawings several forms which are presently preferred, it being understood, however, that the invention is not limited to the precise arrangements and instrumentalities shown.